Thai King Bhumibol Adulyadej is in hospital after feeling weak in his right side, the royal palace has said.

The 79-year-old king was taken to Bangkok's Siriraj hospital on Saturday morning, where scans revealed an inadequate blood supply to his brain.

The condition improved after treatment but requires further observation.

King Bhumibol, the world's longest reigning current monarch, is greatly revered in Thailand and his health is a subject of great national concern.

A statement from the Bureau of the Royal Household said scans had revealed "a minimal cerebral ischemia" on the left side of the king's brain, meaning that part of his brain was not receiving enough blood to function normally.

"The team of royal physicians then recommended hospitalisation for treatment and observation," the statement said.

"After a period of approximately eight hours of treatment, the weakness of the right leg has improved," it added.

Concerns over King Bhumibol's health have deepened since he underwent spinal surgery last year, after complaining of back pain.